(1) Field of the Invention
The present invention relates to a supercharge pressure control method and apparatus for vehicles having a turbocharger and an internal combustion engine supercharged by the turbocharger.
(2) Description of the Prior Art
It has been widely practiced heretofore that intake air is supercharged by a turbocharger in order to improve the output power of the internal combustion engine.
The turbocharger comprises an exhaust gas turbine driven by the exhaust energy of the engine, and an intake air compressor capable of being rotated by the exhaust gas turbine for supercharging the intake air. The exhaust gas turbine is, on the other hand, rotated at high speed when the exhaust gas flow rate of the engine increases and it can drive the compressor efficiently. However, in a low load operation zone of the engine having a small exhaust gas flow rate, it is inevitable that the supercharge efficiency of the exhaust gas turbine lowers.
For the purpose of preventing the number of rotations of the exhaust gas turbine from lowering and for efficiently supercharging the turbine even in the relatively small engine load zone, a supercharge pressure apparatus has already been proposed in which a control valve is provided at the inlet of the exhaust gas turbine for controlling the exhaust gas flow speed, and the opening of the control valve is controlled in accordance with the operating conditions of the engine, thereby reducing the opening of the control valve in a low load operation zone having less exhaust gas flow, so as to increase the exhaust gas flow and meet the purpose (see, for instance, U.S. Pat. No. 2,944,786).
In this kind of the supercharge pressure apparatus, however, the supercharge pressure is increased with minimum opening of the control valve when the engine load is low, so as to increase the output power of the engine. Accordingly, in a vehicle having an automatic transmission, when the transmission is suddenly changed from a neutral or neutral position to a running range or running position during the idling condition of the engine or when the vehicle is changed from low speed running condition to a suddenly accelerated condition, a large drive torque tends to be suddenly transmitted to the power train. Namely, the large drive torque from the engine tends to be suddenly transmitted from the output side of the transmission to the power train such as the differential gear mechanism of the wheels via a propeller shaft, with the gears of the transmission maintained at a low speed position. As a result, the power train temporarily becomes overloaded and there occurred a problem that durability of the power train was often reduced.